The present invention relates in general to polyester with caustic material mixing methods and mixing reactors and, in particular, to methods and mixing reactors in which polyester, for example, polyethylene terephthalate (PET), is mixed with a caustic material such as, for example, an alkaline composition.
In particular, the invention relates to such methods and mixing reactors that can be effectively employed in cleaning, decontaminating, and even enhancing the intrinsic viscosity of polyesters. As such, these methods and mixing reactors can be useful in a variety of processes, including separating materials such as waste materials, which contain impurities and/or contaminants, from polyesters and solid-stating of polyesters.
A polyester is a polymeric material, which can be made from the esterification of polybasic organic acids with polyvalent acids. Perhaps the most commonly made and used polyester is polyethylene terephthalate (PET), which can be manufactured by reacting terephthalic acid with ethylene glycol.
Polyesters are currently being used in increasing amounts in various applications. For instance, polyesters are commonly used to make all types of containers, such as beverage and food containers, photographic films, X-ray films, magnetic recording tapes, electrical insulation, surgical aids, such as synthetic arteries, fabrics and other textile products, and many other items.
A challenge in the field of polyester technology is the recovery and recycling of polyesters. Since polyesters can be economically remelted and reformed, many efforts are underway to recycle as much polyester as possible after use. Before polyesters can be recycled, however, it is necessary to separate the used polyesters from other products and materials that may be found mixed with or attached to the polyester. In the current state of the art, many problems have been encountered in attempting to separate polyester from other waste materials. In particular, many prior art processes are not capable of efficiently or economically recovering polyester, when a significant amount of other materials, impurities, and contaminants are present.
The majority of the prior art processes for separating polyesters from other materials have focused on “float”-separation techniques and mechanical recovery processes. In the float-separation technique, polyesters are separated from other materials based on density differences. For instance, materials containing polyester can be combined with water. The less dense materials that float on the water can be easily separated from the submerged polyester. This procedure can be effective in separating polyesters from many low density impurities. However, float-separation techniques cannot be used, if the polyester is found in combination with materials that also sink in the water or that have densities that are comparable to that of polyester.
Examples of the latter include polyvinyl chloride (PVC) and aluminum, both of which do not float in water. In fact, PVC has a density that is very similar to the density of PET and is, therefore, often misidentified as PET. However, both aluminum and PVC must be separated from polyester before it can be reused. In particular, if PET and PVC are remelted together, hydrochloric acid gases are produced that destroy the properties of the resulting plastic material.
Besides failing to separate polyesters from impurities that are heavier than water, the float-separation techniques and conventional washing also fail to remove coatings or other contaminants that commonly adhere to polyester. For example, polyester containers are commonly coated with vapor barrier coatings, saran coatings, and/or inks.
Mechanical recovery processes typically involve washing processes that are used to strip the surface coatings and contaminants off the polyester without any substantial reaction occurring between the polyester and the wash solution. For example, U.S. Pat. Nos. 5,286,463 and 5,366,998 disclose a composition and process for removing adhesives, in particular resins, based on polyvinylidene halide and polyvinyl halide, from polyester films, such as photographic films. In one process the polyester films are mixed with a reducing sugar and a base, in order to remove the adhesive polymeric resin from the film. Then an acid is added to precipitate the resin, which can then be separated from the polyester film.
Meanwhile the focus of recovering polyester from the recycling of waste has turned to the chemical conversion of the polyester into usable chemical components. In such processes alkaline materials have been employed. For instance, U.S. Pat. Nos. 5,395,858 and 5,580,905 disclose processes for recycling polyesters. In these processes the polyesters are reduced to their original chemical reactants.
This process includes the steps of combining the polyester material with an alkaline composition, in order to form a mixture. The mixture is heated to a temperature that is sufficient to convert the polyester to an alkaline salt of a polybasic organic acid and a polyol. During the process the alkaline composition is added in an amount sufficient to react with all of the polyester that is present in the mixture. The foregoing process provides for the complete chemical conversion/saponification of the polyester material. However, this can add a substantial cost to the overall process, since the polyester must ultimately be reformed.
U.S. Pat. No. 3,590,904 discloses a process for removing polymeric sublayers from polyester film bases, which bear at least one vinylidene halide co-polymeric layer on at least one surface of the base, by making contact with the polyester base material with hot alkaline glycol, in which up to 5 percent by weight or more of water can be present, in order to induce a reaction between the hot glycol and the substratum layers for cleaning the polyester.
WO 00/188830 A1 discloses that a reaction of polyesters, such as PET, with alkaline compositions in a reactive environment that is at least substantially free of water can allow the cleaning, decontamination and even an improvement in the intrinsic viscosity of polyesters. This innovation is in direct contrast to a long-held belief in the field of polyester recycling that polyesters must necessarily degrade in the presence of caustic materials.
In one aspect WO 00/18830 A1 relates to a method for treating a polyester, which includes the combining of a polyester with an alkaline composition in an amount, which is effective, in order to coat at least a portion of the polyester. Then during the actual reaction step the polyester can be heated to a temperature which is not greater than the melting point of the polyester. The aforementioned actual reaction step is conducted in an environment that is at least substantially free of water.
This technique, which is known in the prior art, can be used in order to improve the properties of polyester products, which are obtained from processes for recovering or recycling polyester containing materials, which contain contaminants and/or impurities. In this regard a recovery method includes mixing the polyester containing material with an alkaline composition, where the alkaline composition is employed in an amount, which is effective in order to react with a portion of the polyester and the polyester containing material, followed by heating the mixture in the actual reaction step to a temperature, which is effective in order to saponify a portion of the polyester, but is insufficient to melt the polyester. Then the polyester can be physically cleaned and decontaminated. A key aspect of this known process is that the actual reaction step is carried out in an environment, which is at least substantially free of water. Depending on, for example, the duration of the process, the resulting polyester may show an improved intrinsic viscosity (i.v.) and color. Prior to reuse, the polyester can be treated by techniques, for example, washing, that are recognized in the state of the art. Since this known technique can be used to enhance the intrinsic viscosity of the polyester, it can also be used, in fact, in a method for forming, for example, solidifying, polyesters.